In belt pretensioners known hitherto comprising a piston/cylinder unit, provided with a pyrotechnical piston drive, the connecting part extends through an opening in the cylinder wall adapted as precisely as possible to the outer diameter of the connecting part, it being necessary thereby to prevent an excessively large gap between the opening and the connecting part via which the generated gas may escape. In this arrangement, the connecting part is either a piston rod or a traction cable having a smooth outer surface as possible. A ring seal may also be provided in the end face opening, said ring seal being intended to prevent escape of the gas. The seals normally used hitherto are designed so that the friction force between the seal and the connecting part is as low as possible and that the connecting part together with the belt deflection member may be easily shifted. Particularly due to the high cylinder internal pressure, when the gas generator is activated and when employing a traction cable as the connecting part, leakage results despite using a seal thus reducing the cylinder internal pressure and the tensioning force provided by the belt pretensioner.